1. Field of the Invention
The present invention relates to an electrophotographic image forming apparatus such as a copier, a printer, a facsimile machine or a complex machine and particularly to an image forming apparatus employing a non-contact developing method for developing an electrostatic latent image using a two-component developer, in which a nonmagnetic toner is charged by a magnetic carrier, by holding only the charged toner on a developing roller and transferring the toner toward the electrostatic latent image.
2. Description of the Related Art
Conventionally, development by a non-contact developing method with the use of a one-component developer has been considered for image forming apparatuses such as copiers, printers, facsimile machine and complex machines. In recent years, with the speeding up of printing, consideration has been made about image development for a high-speed image forming method, particularly image development for a one-drum color superimposing method for successively forming a plurality of color images on a photoconductor. By this one-drum color superimposing method, color image formation with less color drift is possible by accurately superimposing toners on the photoconductor, and this method is attracting attention as technology for coping with higher quality of color images.
Recently, attention has been drawn to a so-called tandem method for forming color images in synchronism with the feed of a transfer member and superimposing them on the transfer member using a plurality of photoconductors corresponding to the respective colors of toners. This method has an advantage of being fast, but has a disadvantage of enlarging the apparatus since electrophotographic processing members (image forming units) of the respective colors have to be arranged side by side. In view of this disadvantage, there has been proposed a small-size tandem image forming apparatus in which image forming units miniaturized by narrowing intervals between photoconductors are arranged.
Concerning this tandem image forming apparatus, technology for image development by supplying a developer to a donor roller (developing roller) by means of a magnetic roller and causing the toner to transfer to the donor roller to form a toner layer is disclosed, for example, in patent literature 1 (U.S. Pat. No. 3,929,098). However, with this technology, the charge control of the toner is complicated and a high surface potential and a large developing electric field need to be applied to the photoconductor.
Further, since it is difficult to remove the toner on the donor roller unused for image development, if a toner consumed region and a toner nonconsumed region are formed on the donor roller, an adhering state of the toner and a potential difference of the toner on this donor roller vary. Thus, there is a problem of the occurrence of a phenomenon in which part of a previously developed image appears as a residual image (ghost) during the next image development, so-called a history phenomenon.
In view of this problem, technology is disclosed, for example, in patent literature 2 (Japanese Unexamined Patent Publication No. 2003-21961) and patent literature 3 (Japanese Unexamined Patent Publication No. 2003-21966) according to which a magnetic roller (magnetic brush roller) for holding a magnetic brush formed using a two-component developer containing a carrier and a toner by a magnetic member fixed inside, a developing roller for forming a toner layer by contact with the magnetic brush and a power supply for forming an alternating electric field between the developing roller and a photoconductor are provided, and a latent image on the photoconductor is developed with the toner transferred from the toner layer by the alternating electric field to prevent the occurrence of a residual image (ghost) during image development while avoiding the occurrence of fogging.
Further, patent literature 4 (Japanese Unexamined Patent Publication No. 2001-134050) discloses technology in a developing device using a one-component developer, including a developing roller held in contact with a photoconductor and a supply roller held in contact with this developing roller, and adapted to supply a toner to the developing roller by means of the supply roller and to form a thin layer of the toner frictionally charged by a restricting blade to develop a latent image on the photoconductor, wherein an alternating voltage is also applied to the supply roller and the both alternating voltages are set to have the same frequency, but different phases.
According to this technology, if a developing electric field applied to the developing roller is an alternating-current electric field in light of preventing a problem that low density images and thin line images are difficult to develop or the occurrence of density nonuniformity caused by an increase of a toner charge amount, low density images and thin line images can be satisfactorily developed and the toner unused for image development can be easily scraped off. However, fogging occurs if an alternating voltage is too high, whereas the effect of pulling back the toner unused for image development is reduced if the alternating voltage is low. This technology seeks to solve this problem.
Further, in order to solve the above problem, patent literature 5 (Japanese Unexamined Patent Publication No. 2005-242281) discloses technology in a developing device in which a toner layer is formed on a developing roller by contact with a magnetic brush formed of a two-component developer and toner is transferred from the developing roller by an alternating electric field of a rectangular wave generated between the developing roller and a photoconductor by a first power supply, thereby developing a latent image on the photoconductor, wherein an alternating electric field of a rectangular wave having the same frequently as, an opposite phase to and an inverted duty ratio of the one generated by the first power supply is applied between a magnetic roller and the developing roller by a second power supply.
However, with the above respective technologies, if Vslv and Vmag denote, for example, a bias voltage (alternating-current bias) to be applied to the developing roller and a bias voltage to be applied to the magnetic roller (magnetic brush), a power supply construction for applying the bias voltages is such that the bias voltages Vslv, Vmag are applied to a developing roller 901 and a magnetic roller 902 respectively by first and second bias power supplies 911, 912 (respective power supplies are individually grounded), for example, as shown in FIG. 6. Thus, a potential difference between the magnetic roller 902 and the developing roller 901 can be obtained as a difference between the bias voltages Vslv and Vmag.
In consideration of the balance of the releasability of toner on the developing roller unused for image development, toner thin layer formation and toner developability between the magnetic roller and the developing roller, optimal alternating bias voltages applied between the magnetic roller and the developing roller are, for example, in the above example such that the bias voltage Vslv applied to the developing roller 901 has a duty ratio of 10 to 30%, a frequency of 4 kHz and a Vpp of 1.6 kV and the bias voltage Vmag applied to the magnetic roller 902 has a duty ratio of 70 to 90%, a frequency of 4 kHz and a Vpp of 0.3 kV.
In the following description, duty ratios are all expressed in percent (%).
However, as the toner particle diameter is decreased for faster image development and higher image quality, a range for maintaining the above balance becomes narrower. Thus, if durability is also considered, it is difficult to ensure optimal values.
Since the potential difference between the magnetic roller and the developing roller is obtained as the difference between the bias voltages Vslv and Vmag as described above, it cannot be directly set, wherefore the potential difference between the magnetic roller and the developing roller needs to be controlled to a desired potential difference by balancing the respective output voltages of the first and second bias power supplies 911, 912.
Since the respective output voltages of the first and second bias power supplies 911, 912 relate to controls of the releasability of toner on the developing roller unused for image development, the toner thin layer formation and the toner developability between the magnetic roller and the developing roller, it is not easy to set the potential difference between the magnetic roller and the developing roller to a desired potential difference while balancing voltages suitable for these controls and the potential difference between the magnetic roller and the developing roller.
For example, patent literature 6 (Japanese Unexamined Patent Publication No. 2003-280357) discloses technology for applying an alternating bias voltage having a duty ratio of 10 to 50% to a developing roller. This technology is for applying the alternating bias voltage only to the developing roller without applying it to a magnetic roller. Particularly, the duty ratios of the alternating bias voltages applied to the magnetic roller and the developing roller are not mentioned at all in patent literature 6.
Further, in this developing method (touch-down developing method: processing by a two-component method up to the magnetic roller and, then, image development is performed by a one-component method for forming a toner thin layer on the developing roller by toner from the magnetic roller and transferring the toner), the toner thin layer is selectively (preferentially) formed by toner particles easier to transfer upon forming the thin toner layer on the developing roller by the transfer of toner particles from the magnetic roller and a charge number distribution of the toner (toner particle distribution) in the two-component developer largely varies between at the start of printing and after repeated print outputs, wherefore problems such as image density defects, fogging and toner scattering occur and it is difficult to maintain stable performances over a long term.
Concerning this, technology for developing an image such that the charge number distribution of toner on a developing roller and that of toner in a developer on a magnetic roller differ is, for example, disclosed in patent literature 7 (Japanese Unexamined Patent Publication No. 2001-272857).
However, that the charge number distributions of the toners on the developing roller and on the magnetic roller differ indicates that toner particles with a specific charge in the two-component developer on the magnetic roller selectively transfer to the developing roller. Specifically, if the toner particles are selectively transferred, the toner charge distribution in the two-component developer broadens, wherefore it becomes difficult to stably form the thin toner layer (image forming operation; printing operation) on the developing roller over a long term.